How to make a user-friendly book index

I've
used books where the indexes drove me nuts. My least favorite was
a field guide that separated the common and scientific names into
different indices --- I always seemed to flip to the wrong index when
in a hurry and then wondered why my word wasn't present.

Even worse is a
non-fiction book without an index, or with an index
less than a page long. When I pick up a non-fiction book and
notice it has no index, I generally put it back down in disgust.

At the other extreme,
I've flipped through indexes that made the
contents of the book not only easy to access but also clearer.
These indexes draw connections between sections I hadn't really
considered, pointing out ideas that all fit within the same
theme.

I'm not going to talk
about the nuts and bolts of actually creating an
index because that's program specific and seems to be pretty
simple. What I'm having a harder time working my head around is
--- what factors will make my index easy to use? How can I create
an eye-opening instead of an eye-rolling index?

Here are a few tips I've
stumbled across:

Decide
on basic formatting first.
For example, most indexes make all words lower case unless they're
proper nouns, but some indexes will capitalize main entries. The
entry should be a plural noun, so if you wanted to index "red
cats", you'd list it as "cats, red." And unless they're
absolutely essential to parsing the entry, prepositions just use up
space and can be deleted.

Think
like a reader and use synonyms.
A great index will allow a reader to find a paragraph they vaguely
remember...even if they can't bring any of your terminology to
mind. So, in my section about the benefits of no-till gardens, I
should keep in mind that readers might not be as familiar with the term
"no-till" as I am. I might add in some index entries for terms
like "tilling, problems with", "rototillers", and "plows." In
addition, it's helpful to index key terms in several different ways ---
a reader might look up any of the following terms when searching for my
kill mulch section, so I'll include them all: "kill mulch", "lasagna
garden", "sheet mulch." In fact, "kill mulch" is an odd term
since a reader might not be entirely sure whether the "kill" is just an
adjective or an essential part of the phrase, so I'd probably index it
both under "kill mulch" and "mulch, kill."

Decide
whether to cross-reference.
There are two options for dealing with all of these synonyms. You
can either say "see kill mulch" under both "lasagna garden" and "sheet
mulch", or can simply insert the relative page references for each
synonym. If "kill mulch" is a main heading with several entries
under it, you'd probably want to cross-reference to save space.
But also be aware that readers have limited attention spans and would
vastly prefer to see what they're looking for immediately without being
sent to
another entry.

Figure
out how deeply you want to index.
Do you want a reader to be able to find that one paragraph on Swiss
chard, or do you think they'd be just as well off if they can easily
find the section on summer vegetables? An in-depth index will
often take up 5% to 10% of the length of your book, including two to
three
columns of terms per page. If your index is clocking in a lot
longer or shorter than that, it's probably either too detailed or too
light to be handy.

Think
of your metatopic.
Your book should have one main thesis that the whole text revolves
around. This metatopic isn't actually included in the index --- I
won't want to have an entry for "homesteading" in the Weekend
Homesteader index
because I'd have to put every section in the book
underneath that main heading. However, knowing what your
metatopic is will help you conceptualize your entire index as
subheadings, giving you an idea of what main headings you want to
emphasize. For my book, primary headings under the metatopic
"homesteading" would include terms like "gardening", "emergency
preparedness", and "cooking."

Choosing
main headings.
Every indexer has a different method of actually creating the index,
but I really liked the idea of simply tagging all of the terms I found
important during the first run through the book, ignoring the fact that
some of them will end up as entries under other headings. Next, I
can
print the index rough draft (or open it in another file) and spend some
time thinking about what sorts of headings could lump terms together
and make them easier to find. Adding
subheadings.
As a
rule
of thumb, you need some subheadings if you have more than three or four
pages listed for a heading. Think of how annoying it would be if
I summed up the sections of my book about heat during power outages,
backup lighting, and storing drinking water as "Emergency
preparedness...5, 34, 71, 85, 103, 150." Isn't it more handy to
see:

Don't
include too many subheadings.
If you've got more than about 15 subheadings under a certain entry,
you're probably trying to fit too much into that topic. For
example, I could list a hundred subheadings under "gardening" if I felt
like it, but who wants to skim through two columns worth of entries
looking for their keyword? There are two ways to deal with
headings that feel too large. You can either completely delete
that heading and make the biggest subheadings into main headings ---
for
example, remove "gardening" and instead have entries for "vegetables",
"no-till", and so forth. Or you can leave some entries under that
main heading, but include a note to "see also mulch" and "see also soil
testing."

Have you run across any
other tips for making indexes user
friendly? Do you have horror stories about the world's worst
index? I'm getting ready to whip out an index on my Weekend
Homesteader book and could use any advice you have.